1. Field of the Invention
The present invention relates to an apparatus for encoding fishfinder image information. The apparatus is used to transmit an image signal indicative of the condition of the sea below the apparatus from a comparatively remote place. The present invention also relates to a method for transmitting encoded information and a transmission system for transmitting a fishfinder image by using said transmission method.
2. Description of the Related Art
Two-dimensional image information has been transmitted to a remote place using a wire system as well as a wireless system. However, fishfinder image information representative of the presence of fish shoals in the sea below a detection device is generally transmitted from ships or buoys by using the wireless system. The so-called fishfinder image information is sufficient if it includes data indicative of the location of fish shoals and of the extent of the fish presence there. Thus, data relayed by the transmission system is not required to be highly accurate or at high speed, and, in practice, the information can be transmitted even in the form of a time-series analog signal.
In general, the so-called fishfinder emits a supersonic wave from a sound transmitter toward fish shoals in the sea below the emitter, receives a group of reflected waves from the sea bottom and the fish shoals, and displays the fishfinder image information on a CRT display unit and recording paper. The information indicative of the presence of fish shoals is of a two-dimensional format so that the brightness of each display spot varies. Thus, to transmit the information through a communication network or wireless system, the information must be converted into a one-dimensional format.
In the prior art, as shown in FIG. 8, the fishfinder image information is subjected to scanning and photoelectric conversion in an image transmitting device 1 so that it is converted from the two-dimensional format to the one-dimensional format. The fishfinder image information in the one-dimensional format is subjected to encoding and modulation in an image signal converting device 2 on the transmit side. After passing through a transmission network 3, the information is received in an image signal converting device 4 on the receiver side and subjected to demodulation and decoding so that it is converted from the one-dimensional format to the two-dimensional format. Finally, the information thus recovered is displayed as the fishfinder image information by an image receiving device 5.
In the image transmission of the above two-dimensional format, conventionally, the information of the fishfinder image is converted from analog to digital form and stored in a memory, then encoded per pixel (dot) before transmission. Therefore, since the information content of the fishfinder image becomes very large and dense, there is a need for a communication network having a very wide bandwidth and high speed and a system of high performance, as compared to the transmission in code form of character information.
Such a transmission system is susceptible to noise. To suppress the influence of noise, the use of a comparatively high frequency band, such as VHF or UHF, would be proposed; but, this limits the attainable distance of communications, increases facility costs and maintenance costs, and renders a transmission system bulky.